There is a tendency to use pre-written libraries of code to save development time. However, these can add significantly to the initial delay when accessing a site, which can critically affect the "bounce rate". For this reason we avoid pre-written libraries as far as possible.
All our sites are tested on Chrome, Firefox, Safari, Internet Explorer (IE8 and current), and mobile devices to ensure consistent performance across a range of browsers and devices. We also use broadband, wifi and 3G links to check the responsiveness of the sites under realistic conditions.